Apparently Starcraft 2’s single player campaign will be split into 3 seperate products. Link to Kotaku. It depends on the pricing and how much we the players are getting from each product. If each product is 50 dollars than It’s already lost, hardly anyone will buy the second two. But if they are treated like expansions packs that add on to the original than it might now be do bad. Say they release the first one with the terran single – player, and also multiplayer for everything. Then x months later they come out with a 20$ addition which is the zerg single-player and maybe even changes plus more maps for the multiplayer than i could easily see that justifying the split into 3 products.
The thing that will decide how well Starcraft 2 sells, is the pricing and the timing. If they decided to release all three products at launch and they all cost the same then theirs no hope, only the absolutely hardcore will purchase it. All i see for the imediate future is consumer backlash that could come back to hurt starcraft 2 a lot.